1. Field
This document relates to an organic light emitting diode display device and a driving method thereof capable of preventing a luminance variation due to an external temperature or ambient light.
2. Related Art
Flat display devices include liquid crystal displays (“LCDs”), field emission displays (“FEDs”), plasma display panels (“PDPs”), electroluminescence devices, and the like.
The PDP has light weight and thin profile, and is advantageous to a large-sized screen because of simple structure and manufacturing process, but is disadvantageous in that luminous efficiency and brightness are low and power consumption is great. The TFT LCD (Thin Film Transistor LCD) is a widely used flat display device, but is disadvantageous in that a viewing angle is small and a response speed is low. The electroluminescence device is largely classified into an inorganic light emitting diode and an organic light emitting diode (“OLED”) according to materials used for an emission layer, which is a light emitting device which emits light for itself, and is advantageous in that a response speed is high, and luminous efficiency, brightness and viewing angle are great.
The OLED display device, as shown in FIG. 1, includes an OLED. The OLED has an anode electrode, a cathode electrode, and organic compound layers.
The organic compound layers include a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When driving voltages are applied to the anode electrode and the cathode electrode, and holes supplied via the hole injection layer HIL and the hole transport layer HTL and electrons supplied via the electron injection hole EIL and the electron transport layer ETL are moved to the emission layer to form the exciton, and in turn the emission layer EML emits light.
In the OLED display device, a plurality of pixels each including the OLED is arranged in a matrix, the pixels are selected by applying scan pluses to TFTs which are active elements, so as to be selectively turned on, and pixel data is supplied to the selected pixels, thereby controlling luminance for the pixels. Each of the pixels includes a driving TFT, at least one switching TFT, a storage capacitor, and the like, and the luminance for the pixel is proportional to a driving current Ioled flowing through the OLED as expressed in Equation 1.
                    Ioled        =                              k            2                    ⁢                                    (                              Vgs                -                Vth                            )                                      2              ↵                                                          Equation        ⁢                                  ⁢        1            
Here, ‘Ioled’ denotes a driving current, ‘k’ is a constant value defined by the mobility of the driving TFT and the parasitic capacitance, ‘Vgs’ denotes a gate-source voltage of the driving TFT, and ‘Vth’ denotes a threshold voltage of the driving TFT.
In the OLED display device, if the mobility of the driving TFT varies due to influence of an external temperature, or a photo current flows through the driving TFT due to influence of ambient light, the driving current Ioled flowing through the OLED varies. In a case where the driving current Ioled flowing through the OLED is greater than a value corresponding to the pixel data, the lifespan of the driving TFT and the OLED is shortened, and in a case where it is smaller than the value corresponding to the pixel data, a contrast ratio is lowered. Therefore, the present applicant has proposed a current feedback algorithm which enables an ideal driving current corresponding to pixel data to flow through an OLED by real-time feedback of a present driving current in Korean Patent Application No. 10-2009-0132960, which was filed. In this technique, a driving current is detected, and a high potential level is adjusted such that the detected driving current value is the same as a driving current value predicted from pixel data. Thereby, the technique prevents luminance from varying due to variations in external environmental conditions such as an external temperature or ambient light, by realizing ideal luminance corresponding to pixel data.
FIG. 2 shows a configuration of a current sensing circuit for detecting a driving current flowing through the OLED in the related art. Referring to FIG. 2, the current sensing circuit includes an operation amplifier 1 which converts the driving current Ioled flowing through the OLED into a voltage and amplifies the voltage, and an ADC (analog to digital converter) 2 which converts the amplified analog voltage value into a digital sensing current value Isen. In FIG. 2, ‘Rs’indicates a sensing resistor connected between the driving voltage supply line 3 and the OLED.
However, in the OLED display device, the pixels emit light by being applied with driving voltages for driving the pixels during the initial driving, and thus a great number of currents may instantly flow through the driving voltage supply line 3. In this case, a voltage from the output terminal of the operational amplifier 1 may exceed an input voltage range of the ADC 2. In the related art, such an over-current may cause damage in the driving circuits and operation errors in the current feedback algorithm during the initial driving.